1. Field of the Invention
This invention relates generally to wireless communication systems, and more particularly, to a low power packet detector for low power wide local area network (WLAN) devices. The invention is particularly useful and relevant to packet based wireless local area networks such as IEEE 802.11-based networks.
2. Description of the Prior Art
Wireless LAN chipsets and equipment have seen a rapid ramp up in revenue and market adoption in the last a few years. So far, the WLAN market has been mostly driven in the corporate market by the laptop users for its convenience of untethered connectivity, and driven in the home market for home networking purposes such as sharing of broadband connections among multiple PCs. The main care-abouts for the laptop and home PC users have been throughput, cost, and coverage. Due to the relatively large capacity of a laptop battery and the low expectations from end users (usually 4-5 hours of uninterrupted usage between charges are acceptable performances), power consumption of WLAN systems has been relatively relaxed compared with their brethrens in cellular wireless markets.
As the WLAN market matures and the cost of WLAN systems rapidly decreases, a lot of interest has been generated regarding the incorporation of WLAN capability into portable devices, notably cell phones and PDAs. Most of these portable devices are battery powered, and end users expect at least a few days of continuous use between battery charges. In addition, the WLAN feature, which provides connectivity for the primary applications such as voice, should not significantly reduce the usage time for the primary applications. The rule of thumb is that a new feature should not reduce battery life by more than 20%. Using the example of a 1500 mAh battery and at least 10 hours of active network connections, one could calculate that the average power consumption for WLAN system should be not more than 100 mW. Considering that today's WLAN system typically has a power consumption of around 700 mW during active usage, there is apparently a lot of work left on the power reduction front.
A more careful study of the usage pattern of most PDA/cell phone data services reveals that the network link is dominated by AP to terminal (downlink) traffic. Most popular applications envisioned for WLAN such as web browsing, email downloading, video streaming and entertainment are all dominated by downlink data traffic. An 802.11b BSS, after removing channel access and collision overhead, has a network maximum capacity between 5-7 Mbps. However, most applications including media heavy applications such as video streaming, rarely require a throughput of more than 1 Mbps at individual terminals. However, the 802.11 MAC protocol is based on the CSMA/CA (carrier sense multiple access with collision avoidance) contention based channel access mechanism, requiring all contending stations decoding all packets on the medium at all time to synchronize and avoid collisions. The side effect is that each WLAN device would be decoding packets at the network throughput, instead of at the data throughput at each terminal. The overhead in power consumption is phenomenal.
In view of the foregoing observations, it is both desirable and advantageous to provide a method that can significantly reduce the effective power consumption of a WLAN terminal device.